Typically, high lift systems of commercial and military aircraft are powered by a centralized drive, also known as a power control unit (PCU), which is mounted in a central region of the fuselage, and is controllable through a computerized control. The PCU is coupled with a torque shaft system, also known as transmission shaft, which transfers mechanical power to geared actuators at flap or slat panel drive stations distributed along the trailing edge or leading edge of a wing, depending on the desired type of high lift surfaces. A wing tip brake, which is coupled with the transmission shaft and particularly placed in an outer region of the transmission shaft and/or in a tip region of the respective wing, is capable of arresting and holding the transmission shaft in certain conditions. The control of the PCU is usually conducted by control computers, such as slat flap control computers (SFCC), which are commonly realized as a redundant arrangement of two independent SFCCs that are not only able to control but also to monitor the operation of the high lift system.
A PCU commonly comprises two independent motors, which are coupled with an output shaft by means of a speed summing differential gear. Each of the motors is provided with a power off brake (POB) for arresting the motor in a commanded position. While at least one of the two motors is commonly a hydraulic motor, the second motor may be realized as a further hydraulic motor, but also as an electric motor, leading to a hybrid PCU. The hydraulic motor may preferably be realized by an axial piston motor comprising a movable swash plate, wherein the orientation of the swash plate inter alia determines the resulting speed of the motor. A solenoid valve may be arranged between a hydraulic network of the aircraft and the respective hydraulic motor, leading to a pressurization of the hydraulic motor, and allows the motor to spin when the swash plate is in a desired position.
Still further, high lift systems usually comprise torque limiters that are adapted for limiting the torque to be introduced into the transmission system. The torque limiters may be mechanical or electronic torque limiters, wherein the latter rely on constantly monitoring an introduced torque, taking authority over the motors of the PCU, and initiating a reversal once the torque exceeds a predetermined threshold.
EP 1 685 026 B1 discloses a device for limiting a load in an aircraft high lift system, the system comprising individual segments of landing flap systems and slat flap systems, and a drive unit, wherein signals from at least two position sensors are measured, a reference variable is calculated form the measured signals and compared with a corresponding threshold value pre-determined from a maximum authorized load, and a control signal is generated for limiting the drive power, when at least one of the reference variables reaches or exceeds the threshold value.
EP 2 727 831 A1 discloses a method for transferring hydraulic power between at least two hydraulic systems in an aircraft comprising the steps of connecting two hydraulic displacement machines to a differential gear unit having a common mechanical output, arresting the common mechanical output, and operating the first hydraulic displacement machine in a motor mode under consumption of hydraulic power of the first hydraulic system such that the second hydraulic displacement machine is mechanically rotated in a pump mode and supplies hydraulic power to the second hydraulic system.